This invention relates to cellulose semipermeable hollow fibers of the type useful in dialysis, osmosis or ultrafiltration type separatory cells, and more particularly cells useful in the detoxification of blood by hemodialysis or hemofiltration. The invention also relates to an improved method for making the new fibers.
In the past the major quantity of cellulose hollow fibers used in artificial kidneys in hemodialysis was made by melt extrusion of a cellulose ester, such as cellulose triacetate, in a continuous process such as the process of U.S. Pat. No. 3,546,209. Another portion of the cellulose hollow fibers were made by the cuprammonium regenerated cellulose process of the type disclosed in an improved form in U.S. Pat. No. 3,888,771. Whereas these basically dissimiliar processes produce cellulose fibers possessing commercially acceptable water permeability (ultrafiltration) and urea permeability (clearance) characteristics for use in artificial kidneys, the fibers nevertherless fail to possess optimum combination permeabilities. For example, cellulose fibers made by the process of U.S. Pat. No. 3,546,209 have lower water permeability than is desirable in fibers that have acceptable clearance characteristics for urea, creatinine, vitamin B.sub.12 and other low molecular weight blood impurities.
Moreover, continous manufacture of cellulose fibers from melt extruded cellulose acetate fibers involves chemical conversion from the thermoplastic polymer cellulose acetate to the non-thermoplastic polymer cellulose by hydrolysis or saponification in an aqueous alkali bath. During this hydrolysis the thin wall, small size fibers are especially sensitive to contact and fragile. Successful manufacture at minimum efficiency to be commercial depends upon maintaining sufficient tensile strength in the fibers throughout the wet processing steps to avoid breakage or damage. Thus, it would be highly desirable to improve the tensile strength characteristics of the fiber, particularly its wet strength during hydrolysis, or conversion from cellulose ester to cellulose, prior to drying and storage and assembly into hemodialyzers or hemofilters.
The concept of preparing semipermeable hollow fibers by melt spinning a placticized polymer composition was developed in the early 1960's and was first described in U.S. Pat. No. 3,423,491; various types of polymers are there described, including cellulose esters, and suitable plasticizers are discussed for use in forming the melt spin compositions with different types of thermoplastic polymers. The cellulose ester class of polymer developed into the favorite commercial polymer, particularly cellulose acetates, and tetramethylene sulfone, commonly called sulfolane, was normally employed as the plasticizer to make the melt spin composition for use in melt spinning cellulose acetate fibers. U.S. Pat. Nos. 3,494,780 and 3,532,527 disclosed improvements in the sulfolane - cellulose acetate melt spinning process of extruding cellulose acetate fibers involving either an after-spin immersing of the spun fiber in a bath containing a mixture of sulfolane and a polyol having a molecular weight below 4,000, or prior to spinning the fiber modifying the sulfolane plasticizer to include a minor amount of a polyol having a molecular weight below about 20,000. These patents also disclosed that polyols were considered to be unsatisfactory for use alone as the plasticizer to form melt spin compositions with cellulose esters, particularly the cellulose acetates.
This invention is based on the unexpected discovery that cellulose ester melt spin compositions which are sulfolane free and which include only certain low molecular weight polyols, or mixtures thereof can be melt spun into fibers that can be hydrolyzed into cellulose fibers which unexpectedly possess greatly improved wet strengths during the conversion from cellulose ester to cellulose. Omitting the sulfolane, previously considered to be necessary, is the key change which enables production of the greatly improved fibers of this invention. The resulting cellulose ester fibers of this invention possess satisfactorily high intrinsic tensile strengths in their as spun form and moreover the spun fibers retain, and in certain instances increase, their intrinsic tensile strengths during the polyol leaching and hydrolyzing, or deacetylation, steps which convert the cellulose ester fiber into a cellulose fiber.